(a) Field of the Invention
The present invention relates to a light-emitting display, a driving method thereof, and a light-emitting display panel. More particularly, the present invention relates to a current programming method in an active matrix display using electroluminescence of an organic material.
(b) Description of the Related Art
An organic electroluminescent (EL) display is a display that emits light by electrical excitation of fluorescent organic compounds. Using the organic EL display, an image is displayed by driving each of N×M organic luminescent cells with voltage or current.
The organic luminescent cell has characteristics of a diode, and in general is called an organic light-emitting diode (OLED). The organic luminescent cell includes an anode (indium tin oxide (ITO) or metal), an organic thin film, and a cathode layer. As shown in FIG. 1, the organic thin film is formed as a multi-layered structure including an emission layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) so as to increase luminescence efficiency by balancing electron and hole concentrations. In addition, it may also include an electron injection layer (EIL) and a hole injection layer (HIL) separately.
Organic EL displays that have such organic luminescent cells are configured as a passive matrix configuration or an active matrix configuration using thin film transistors (TFTs) or metal-oxide semiconductor field-effect transistors (MOSFETs). In the passive matrix configuration, organic luminescent cells are formed between anode lines and cathode lines that cross (i.e., cross over) each other, and the organic luminescent cells are driven by driving the anode and cathode lines. In the active matrix configuration, each organic luminescent cell is connected to a TFT usually through a pixel electrode and is driven by controlling the gate voltage of the corresponding TFT. The active matrix method may be classified as a voltage programming method and/or a current programming method depending on the format of signals that are applied to the capacitor so as to maintain the voltage.
Referring to FIGS. 2 and 3, a conventional organic EL display of the voltage and current programming methods will be described.
FIG. 2 illustrates a pixel circuit pursuant to the conventional voltage programming method for driving an organic EL element. FIG. 2 illustrates one of the N×M pixels as an example. A p-channel transistor M1 is connected to an organic EL element OLED to supply a current for emission from a voltage source VDD, and the current of the transistor M1 is controlled by a data voltage applied through a switching transistor M2. A capacitor C1 for maintaining the applied voltage for a predetermined time is connected between a source of the transistor M1 and a gate thereof. A gate of the switching transistor M2 is connected to a scan line Sn, and a source thereof is connected to a data line Dm.
When the switching transistor M2 is turned on in response to a select signal applied to the gate of the switching transistor M2, a data voltage from the data line Dm is applied to the gate of the transistor M1. The current IOLED, corresponding to the voltage VGS charged between the gate and the source of the transistor M1 by the capacitor C1, flows to the drain of the transistor M1, and the organic EL element OLED emits light corresponding to the current IOLED. In this case, the current IOLED flowing to the organic EL element OLED is expressed in Equation 1.
      Equation    ⁢                  ⁢    1    ⁢          :                  I      OLED        =                            β          2                ⁢                              (                                          V                GS                            -                              V                TH                                      )                    2                    =                        β          2                ⁢                              (                                          V                DD                            -                              V                DATA                            -                                                                V                  TH                                                                      )                    2                    
where IOLED is a current flowing to the organic EL element OLED, VGS is a voltage between the source and the gate of the transistor M1, VTH is a threshold voltage at the transistor M1, VDATA is a data voltage, and β is a constant.
As expressed in Equation 1, the current corresponding to the applied data voltage is applied to the organic EL element OLED, and the organic EL element emits light with a brightness corresponding to the applied current. The applied data voltage has multiple-stage values within a predetermined range so as to display gray scales.
However, it is difficult for the conventional pixel circuit of the voltage programming method to obtain a wide spectrum of gray scales because of deviations of the threshold voltage VTH of the TFT and electron mobility caused by non-uniformity in the manufacturing process. For example, for driving a TFT in the pixel circuit by supplying a 3V voltage, the voltage is to be applied to the gate of the TFT at 12 mV (=3V/256) intervals to express 8-bit (256) grays. If the deviation of the threshold voltage at the TFT caused by the non-uniformity of the manufacturing process is greater than 100 mV, it becomes difficult to express a wide spectrum of gray scales. It is also difficult to express a wide spectrum of gray scales because β in Equation 1 becomes differentiated due to deviation of the electron mobility.
However, if the current source can supply substantially uniform current to the pixel circuit over the whole data line, the pixel circuit of the current programming method generates substantially uniform display characteristics even when a driving transistor in each pixel has non-uniform voltage-current characteristics.
FIG. 3 shows a conventional pixel circuit of the current programming method for driving an organic EL element, illustrating one of the N×M pixels as an example. In FIG. 3, a transistor M1′ is connected to an organic EL element OLED to supply the current for emission to the OLED, and the current of the transistor M1′ is set to be controlled by the data current applied through a transistor M2′.
First, when the transistors M2′ and M3′ are turned on according to a select signal from a scan line Sn, the transistor M1′ is diode-connected, and the capacitor C1′ is charged by the data current IDATA so that the gate voltage of the transistor M1′ is reduced and the current flows from the source to the drain of the transistor M1′. When the capacitor C1′ is charged so that the drain current of the transistor M1′ is the same as the drain current of the transistor M2′, i.e., the data current IDATA, the charging of the capacitor C1′ is stopped. As a result, a voltage corresponding to the data current IDATA from the data line Dm is stored in the capacitor C1′. Next, the select signal from the scan line Sn becomes a high level voltage to turn off the transistors M2′ and M3′, and an emit signal from a scan line En becomes a low level voltage to turn on the transistor M4′. Voltage is then supplied from the voltage source VDD, and the current corresponding to the voltage stored in the capacitor C1′ flows to the organic EL element OLED to emit light. In this case, the current flowing to the organic EL element OLED is expressed in Equation 2.
      Equation    ⁢                  ⁢    2    ⁢          :                  I      OLED        =                            β          2                ⁢                              (                                          V                GS                            -                              V                TH                                      )                    2                    =              I        DATA            
where VGS is a voltage between the source and the gate of the transistor M1′, VTH is a threshold voltage at the transistor M1′, and β is a constant.
As expressed in Equation 2, because the current IOLED flowing to the organic EL element is matched with the data current IDATA in the conventional current pixel circuit, an organic EL panel has substantially uniform characteristics when a programming current source is uniform over the organic EL panel. However, because the current IOLED flowing to the organic EL element is a micro-current, it takes a long time to charge the data line in order to control the pixel circuit using the micro-current IDATA. For example, if the load capacitance of the data line is 30 pico farads (pF), it takes several milliseconds to charge the load of the data line with the data current of about several tens to several hundreds of nano amperes (nA). Taking a long time to charge the data line is problematic because the charging time is not sufficient (i.e., too long) when considering the data line time of several tens of micro seconds (μs).